Pyrolysis furnaces, also known as cracking furnaces, have been used to crack a variety of hydrocarbon feedstocks to ethylene and other valuable olefinic fluids. Cracking at short residence time and high temperature selectively favors the production of ethylene. In order to maximize the conversion and the selectivity of such cracking conversion of the saturated hydrocarbon feedstock into ethylene, for example, it is desirable to input that quantity of heat needed to effect cracking of the saturated hydrocarbon feed very rapidly while minimizing the time that the initial cracking product is exposed to this quantity of cracking heat to fast crack the saturated hydrocarbon feed to ethylene and then quickly remove the formed ethylene from this high heat environment maximizes the final yield of ethylene for the degree of conversion obtained.
Each cracking furnace has a refractory lined firebox containing a multiplicity of high alloy metal or ceramic cracking lines, composed of process pipes, through the interior passages of which flows the hydrocarbon feedstock to be cracked. Refractories are classified as basic, high aluminum, silica, fireclay and insulating. Special refractories include silicon carbide graphite, zircon, zirconia, and fused cast, among others. Refractory lining may be formed of bricks, castables, or thermal ceramic fiber to cover the interior of the firebox. A suitable amount of diluting steam may be included in the process. Burners are located on the floor and/or walls of the firebox to provide the heat necessary. The heat transfers through the metallic/ceramic materials of the reaction lines into the hydrocarbon feedstock that flows within the reaction lines. Known metallic cracking lines may be as long as 2000 feet and may be coiled in a serpentine shape that runs vertically up and down in the firebox or it may be as short as 40 feet in a straight single pass through the firebox.
The process tubes are heated using convection heating or a combination of convection and radiant heat. In the hot box of an ethylene cracker, for example, the process tubes inside the furnace are operated at temperatures from about 800° C. to about 1150° C., typically from about 950° C. to 1100° C. The tube may be made of a metal alloy or ceramic. Alloys include most stainless steel, cast alloys, wrought alloys, carbon steel and the like, which are well known to those skilled in the art.
The refractory materials, both hard and ceramic fiber, incorporated into the firebox contain the heat permitting the process tubes to be heated for the cracking or related reaction to occur. The fireboxes themselves can deliver an increased radiant heat load to the process tubes. A long process tube length is required to accomplish the desired quantity of heat input into the hydrocarbon mass flow for the short time of residence of this hydrocarbon mass within the metallic/ceramic cracking process tubes. Otherwise, the time of residence of the hydrocarbon mass including the newly generated product within the metallic reaction cracking line must be increased.
Sometimes, the process tubes have fins, channels, and the like disposed along the inner surfaces of the process tube to disturb the fluid flow through the inner tube. Other structures of process tubes may have fins, ribs, and the like disposed along the outer surfaces of the tube to disturb the fluid flow about the outer surface of the tube. U.S. Pat. No. 7,128,139 issued on Oct. 31, 2006 assigned to Nova Chemicals (International) S.A. discloses process tubes having external ribs which are evenly spaced. Adding ribs to the external surface of vertical tubes provides an enhancement to the heat transfer and increases the heat transfer to the process tubes. The product is produced within the tubes. U.S. Pat. No. 7,169,292 issued on Jan. 30, 2007 and assigned to LG Chem, Ltd. discloses pyrolysis tubes have a plurality of mixing blades made by twisting two ends of a plate in opposite directions included within the tubes to improve the yield of product. U.S. Pat. No. 4,847,051 issued on Jul. 11, 1989 and assigned to International Fuel Cells Corporation discloses a reformer tube heat transfer device in which the tubes have a mounting sleeves about individual catalyst tubes which define a helical hot gas flow paths on the outside of the catalyst tubes.
Production of ethylene (ethene) and other olefinic fluids is an energy intensive process because of the reactions and heat needed in the process tubes. Traditional process tubes are fabricated from cast or wrought, high alloy stainless steels. Coke layers form along the inner surfaces of the process tubes during normal operation resulting in reduced mass flow through the tube and a reduction in heat transfer through the sides of the tubes. Additional formation of metal carbides along the tube walls, referred to as carburization, further reduces the structural life of the furnace tubes. The furnace must be periodically shut down in order to remove the deposits of coke. This factor results in a substantial loss of production and facility downtime. Furthermore, coke is an excellent thermal insulator requiring higher temperatures resulting in higher fuel costs and shorter tube life.
Process tubes resistant to coking and carburization are desirable. Efforts to produce such resistant process tubes have concentrated on developing a variety of new alloys that are resistant to carburization and reduce the development of coke layers. Some of these efforts have concentrated on layering different alloys on the tubes. Research has shown that, like aluminides, aluminum and silicon containing alloys, iron, chrome, and certain other alloys also prevent coking and carburization. Ceramic and alloys containing silicon have also been found potentially useful.
U.S. Pat. No. 6,830,676 issued on Dec. 14, 2004 and assigned to Chrysalis Technologies Incorporated teaches coking and carburization resistant iron aluminides for hydrocarbon cracking. In this invention, the cracking tubes have a lining of iron aluminide alloy which is fouling and corrosion resistant. U.S. Patent Application No. 2005/058,851 discloses a composite tube for an ethylene pyrolysis furnace and methods of manufacture and joining same wherein the tube comprises an outer shell made from a wrought or cast heat resistant alloy and an inner core made from another alloy whose composition approximates a powder form. Both outer shell and inner core may be extruded to form the process tubes.
U.S. Pat. No. 5,873,951, assigned to Alon, Inc., issued on Feb. 23, 1999 shows a diffusion coated ethylene furnace process tube in which the inner surface of the process tubes are diffusion coated with a sufficient amount of chromium or chromium and silicon to form a first coating having a thickness of at least two mils which is then cleaned, neutralized and grit blasted. A second coating of aluminum or aluminum and silicon is then diffused onto the first coating to form a total coating thickness of at least five mils; the second coating is also cleaned and polished to provide a smooth uniform surface. Reportedly, less coking occurs in these coated tubes.
U.S. Pat. No. 6,139,649, also assigned to Alon, Inc., issued on Oct. 31, 2000, teaches a diffusion method for coating high temperature nickel chromium alloy products which produces ethylene furnace process tubes having a high temperature nickel chromium alloy product coated on the inner surface thereof. The inner coating has a first layer of chromium or chromium and silicon covered by a second layer of aluminum, magnesium, silicon and manganese which in turn is covered by a third layer of rare earth metals such as yttrium and zirconium. After each layer is applied the tube is heat treated, and finally after the final layer has been applied the final surface is treated with argon and nitrogen to stabilize the surface oxides, and can be polished to minimize sites for carbon buildup. Less coking occurs in these coated process tubes.
U.S. Pat. No. 6,537,388 issued on Mar. 25, 2003 and also assigned to Alon, Inc. discloses a surface alloy system conversion for high temperature applications in which chromium, silicon, aluminum, and optionally manganese are diffused onto the surface of a high temperature ally product, to provide a coating having improved resistance to carburization and catalytic coke formation.
U.S. Pat. No. 6,337,459 issued on Jan. 8, 2002 and assigned to Daido Tokushuko Kabushiki Kaisha teaches a multi-layered anti-coking heat resisting metal tube and a method of manufacturing the process tube in which a preferably powdered alloy is applied to the inner and/or outer surface of the process tube.
PCT application International Publication No. WO2007/064288 published on Jun. 7, 2007, and applied for by Sandvik Intellectual Property AB discloses a metallic tube for heating a medium or subject outside or inside thereof by heat transfer thorough the walls of the tube in which a layer of essential Al2O3 is formed on the surfaces thereof when heated to at least 750° C. At least one of the external and the internal surface of the tube is coated by one of a metal, metal alloy and metal compound, which after oxidation forms as a layer having an emissivity coefficient exceeding 0.7 or by a layer essentially consisting of a metal oxide which has an emissivity coefficient exceeding 0.7.
Although, the majority of reaction cracking lines are comprised of metallic tubes, alternative compositions of the reaction cracking lines are possible. For example, U.S. Pat. No. 6,312,652 issued on Nov. 6, 2001 and assigned to Stone & Webster Engineering Corp. discloses a ceramic drip pipe and tube reactor for ethylene production. The reaction lines of the furnace are fabricated of a ceramic refractory feed inlet pipe coaxially located with a ceramic refractory tube to define an annular space therebetween which is, in part, located without and within the radiant heating firebox volume of such furnace, this to provide for a zone wherein hot cracked olefin product gas is quenched in temperature in such annular space outside of the firebox and a cracking zone within the firebox within which hydrocarbon feed is cracked to an olefin containing product gas composition. The ceramic refractory material construction of the '652 patent permits such a pipe-tube reaction line structure to be exposed to a much greater heat/temperature content of which the firebox is capable than reaction lines of conventional metallic construction. Cracking predominantly occurs within the annular space, meaning that the cylindrical ceramic refractory structures may be of diameters sufficient to provide for high strength structures. Greater firebox temperatures allow the use of a shorter reaction line structure.
U.S. Pat. No. 6,497,809 issued on Dec. 24, 2002 and assigned to Phillips Petroleum Company discloses a method for prolonging the effectiveness of a pyrolytic cracking tube treated for the inhibition of coke formation during cracking in which the tubes have tin and silicon deposited on the surface exposed to the hydrocarbon feed functioning as an antifoulant for inhibiting the formation of coke to desulfurize a sulfur-containing feedstock.